Process for preparing silver catalyst

ABSTRACT

An improved silver catalyst for the oxidation of ethylene with molecular oxygen is made by impregnating a porous support with a silver salt of a neo acid; subjecting the impregnated support to a low temperature activation by heating at a temperature in the range of 250° C. to 300° C. on a moving belt in an atmosphere containing less oxygen than air, and post impregnating the support with an alkali metal, preferably cesium.

This is a continuation of application Ser. No. 08/024,247, filed Mar. 1,1993, now U.S. Pat. No. 5,525,740.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to a supported silver catalystuseful for the vapor-phase oxidation of ethylene to ethylene oxide. Moreparticularly, the present invention relates to a method of preparing animproved supported silver catalyst post impregnated with cesium.

2. Related Art

The use of supported silver catalysts for the oxidation of ethylene toethylene oxide has been long known in the art. Additionally, over theyears various promoting metals have been added to further enhanceperformance. In particular, the use of alkali metals has been disclosedin various amounts and added by different methods. A very extensivereview of the patent literature is given in G.B. No. 2,043,481A. Suchdisclosures have been somewhat inconsistent in their teachings, as canbe seen by comparing U.S. Pat. No. 2,238,474 in which sodium and lithiumhydroxides were suggested as promoters and potassium and cesium wereshown to be poisons to U.S. Pat. No. 2,671,764 where rubidium and cesiumsulfates were suggested as promoting compounds.

Although alkali metals were suggested generally in the earlierdisclosures, it is also generally true that more recent workers in thefield have considered potassium, rubidium, and cesium as the preferredalkali metals. For example, see the series of patents to Nielson, etal., in which these materials were used in small amounts co-depositedwith the silver--U.S. Pat. Nos. 3,962,136; 4,010,115, and 4,012,425.Still more recently the art has emphasized synergistic combinations ofthe alkali metals. For example, see G.B. No. 2,043,481A cited above andU.S. Pat. Nos. 4,212,772 or 4,226,782. The art teaches, in addition,that the alkali metals may be used to rejuvenate used catalysts, as forexample U.S. Pat. Nos. 4,123,385; 4,033,903; 4,177,169; and 4,186,106.The art teaches that the alkali metals may be deposited either beforethe silver is placed on the support (pre-deposited)--U.S. Pat. No.4,207,210; at the same time the silver is deposited (co-deposited)--U.S.Pat. Nos. 4,066,575 and 4,248,741; or subsequent to deposition of thesilver (post-deposited)--G.B. No. 2,045,636A.

The amount of alkali metal was suggested to be in quite a wide range inthe older art. It was often indicated that large quantities, e.g. up toseveral per cent of an alkali metal could be used. More recently, theart generally has taught that small quantities of alkali metals producethe optimum effect no matter when the silver and the alkali metals weredeposited. Kilty in U.S. Pat. No. 4,207,210 related the optimum amountof alkali metal to the surface area of the support. Exceptions to theabove include patents issued to ICI which teach the use of large amountsof sodium alone (G.B. No. 1,560,480) and potassium in combination withsmaller amounts of rubidium and cesium (U.S. Pat. No. 4,226,782).However, the art generally teaches that the optimum will be found insubstantially lower quantities, perhaps on the order of 50-500 ppm byweight.

It has long been recognized that the method of preparing the catalystaffects its performance. The differing heat "reactivations" bear witnessto this. Additionally, the impregnating solutions used and theintermediate steps have been found to effect the final catalyst. Forexample, Winnick in commonly assigned U.S. Pat. No. 4,066,575 disclosesan impregnating solution containing silver lactate, lactic acid, bariumacetate, hydrogen peroxide and water. As a class the lactate basedcatalyst are very stable but exhibit low selectivity. The support isimpregnated with the solution and then first activated by heating in aninert atmosphere at 350° C. for and then dried in air at 200° C. for 12hours. The "activated" catalyst is then impregnated with a cesiumsolution and dried in air at 130° C. for 3 hours. The use of the inertatmosphere during the activation step produced a catalyst that was moreselective, but much less stable, i.e., the catalyst lost its activityfairly quickly resulting in shorter run length for a given end of runtemperature.

Armstrong, in commonly assigned U.S. Pat. No. 4,555,501 disclosed usingan impregnating solution containing the silver salt of a neo acid. Theimpregnated support was then "activated" at temperatures of about 200°C. to 600° C. in the presence of air or reduced oxygen atmospheres, thepresence of some oxygen being desirable. The alkali metal, if desired,was then deposited in small quantities (in the range of 260 wppm).

Cesium now appears to be the preferred alkali metal. Various sources ofcesium are catalogued in the prior art, for example, cesium hydroxide,cesium nitrate, cesium chloride, cesium chlorate, cesium bicarbonate,cesium carbonate, and other anion functionalities such as formates,acetates and the like.

U.S. Pat. No. 4,374,260 discloses the coprecipitation of silver andcesium salt, such as the carbonate from a silver carboxylate/aminocomplex.

U.S. Pat Nos. 4,350,616 and 4,389,338 both show the deposition of CsCO₃onto activated silver catalyst from alcohol solution where the silverwas derived from aqueous silver salt solution.

U.S. Pat. Nos. 4,066,575 and 4,033,903 disclose the preparation ofsilver catalyst from both aqueous and non aqueous salt solutions andsubsequent treatment of the activated silver catalyst with postdeposition of an alkali metal salt such as cesium and anions includingcarbonate from lower alcohol and preferably from aqueous solutions.Similarly U.S. Pat. No. 4,342,667 discloses the post deposition ofcesium on to silver catalyst derived from aqueous solutions.

What is most clear is from the prior art relating to post depositionalkali metal is the general interchangeability of aqueous and nonaqueous procedures, i.e. silver catalyst may be prepared by eitheraqueous or non aqueous procedures and the post deposition of alkalimetal may be aqueous or non aqueous. Furthermore, the salt of silver oralkali metal is not specific. Generally the procedures tended to favorthe presence of water.

What has now been found is that water at any stage and in any amount isdetrimental to the performance of the final catalyst. Thus, the presentpreparation is characterized as being substantially anhydrous with postdisposition of cesium.

It is an advantage of the present invention that catalysts ofexceptional stability in use for the preparation of ethylene oxide areproduced, which have high selectivity at high conversions for theethylene oxide process.

SUMMARY OF THE INVENTION

Briefly stated one aspect of the present invention is a catalystprepared by the process of impregnating a porous support having a lowsurface area with a hydrocarbon solution of a silver salt of an organicacid which is substantially free of water and acid. The impregnatedsupport is subjected to a low temperature activation in an atmospherecontaining less oxygen than air, preferably an inert atmosphere, byheating at a temperature not exceeding 300° C. preferably in the rangeof 250° C. to 300° C. on a moving belt. The activation produces asupport containing the activated silver.

The catalyst is made by impregnating a porous support, preferably havinga surface area in the range of 0.2 to 2.0 m² /g, with a hydrocarbonsolution of a silver salt of an organic acid. The solution should besubstantially free of both water and acid as this aspect has been shownto be especially beneficial to catalyst performance and hence preferred.The impregnated support is activated as described above.

In order to modify the activated silver catalyst an alkali metal,preferably cesium, is added. Stability as that term is used hereinrelates to the temperature in the catalyst bed in operation as afunction of time (time trend).

Another aspect of the invention the activated silver catalyst is ananhydrous post impregnated with an alkali metal, preferably cesium, toproduce a finished catalyst by immersing the support in a circulatingstream of the alkali metal in an anhydrous solvent such as ethanol. Theoptimum amount of alkali metal(s) added will be selected to optimizecatalyst performance and will be dependent upon the surface area of thesupport chosen. That is, more alkali metal will be used on supportswhich have larger surface area than on those having relatively smallsurface area.

The catalyst of the present invention may be employed under oxidizingconditions typical to the art for preparing ethylene oxide by the vaporphase oxidation of ethylene with improved results, especially catalyststability.

The term "inert" as used herein means any gaseous material under theconditions of activation which does not react with silver or any othercomponent of the silver impregnated support. Preferred inert materialinclude nitrogen, helium and carbon dioxide, but other specific materialinclude neon, argon, and the like may be used. The limitation of oxygenduring the activation is of principal concern.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a scanning electron micrograph of a silver catalyst in whichthe silver was deposited by heating in air at 500° C.

FIG. 2 is a scanning electron micrograph of a silver catalyst in whichthe silver was deposited by heating in air at 300° C.

FIG. 3 is a scanning electron micrograph of a silver catalyst in whichthe silver was deposited by heating in an inert atmosphere at 500° C.

FIG. 4 is a scanning electron micrograph of a silver catalyst in whichthe silver was deposited by heating in an inert atmosphere at 300° C.

FIG. 5 is a scanning electron micrograph of a silver catalyst in whichthe silver was deposited by heating in an inert atmosphere at 300° C.and thereafter used about 1000 hours in an ethylene oxide reactor.

FIG. 6 is a scanning electron micrograph of a silver catalyst in whichthe silver was deposited by heating in air at 500° C. and thereafterused about 1000 hours in an ethylene oxide reactor.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENT

Preferred catalysts prepared in accordance with this invention containup to about 20% by weight of silver, expressed as metal, deposited uponthe surface and throughout the pores of a porous refractory support.Silver contents higher than 20% by weight of total catalyst areeffective, but result in catalysts which are unnecessarily expensive.Silver contents, expressed as metal, of about 5-13% based on weight oftotal catalyst are preferred, while silver contents of 8-12% areespecially preferred.

Catalysts may be made with supports comprising alumina, silicasilica-alumina or combinations thereof. Preferred supports are thosecontaining principally alpha-alumina, particularly those containing upto about 15 wt % silica. Especially preferred supports have a porosityof about 0.1-1.0 cc/g and preferably about 0.2-0.7 cc/g. Preferredsupports also have a relatively low surface area, i.e. about 0.2-2.0 m²/g, preferably 0.4-1.6 m² /g and most preferably 0.5-1.3 m² /g asdetermined by the BET method. See J. A. Chem. Soc. 60, 309-16 (1938).Porosities are determined by the mercury porosimeter method; see Drakeand Ritter, "Ind. Eng. Chem. Anal. Ed.," 17, 787 (1945). Pore and porediameter distributions are determined from the surface area and apparentporosity measurements.

For use in commercial ethylene oxide production applications, thesupports are desirably formed into regularly shaped pellets, spheres,rings, etc. Desirably, the support particles used have "equivalentdiameters" in the range from 3-10 mm and preferably in the rang of 4-8mm, which are usually compatible with the internal diameter of the tubesin which the catalyst is placed. "Equivalent diameter" is the diameterof a sphere having the same external surface (i.e. neglecting surfacewithin the pores of the particle) to volume ratio as the supportparticles being employed.

The silver is added to the support by immersion of the support into asolution containing a silver salt of an organic acid which issubstantially free of water and said acid. The silver containing liquidpenetrates by absorption, capillary action and/or vacuum into the poresof the support. A single immersion or a series of immersions, with orwithout intermediate drying, may be used, depending in part upon theconcentration of the silver salt in the solution. To obtain catalystshaving silver contents within the preferred range, suitable impregnatingsolutions will generally contain from 5-50 wt % silver, expressed asmetal, but supplied as silver salts of acids. The exact concentrationsemployed, of course, will depend upon, among other factors, the desiredsilver content, the nature of the support, the viscosity of the liquid,and solubility of the acid silver salt.

Impregnation of the selected support is achieved in a conventionalmanner. The support material is placed in the silver solution until allof the solution is absorbed by the support. Preferably the quantity ofthe silver solution used to impregnate the porous support is no morethan is necessary to fill the pore volume of the porous support.

The impregnating solution, as already indicated, is characterized as asubstantially water free and acid free organic solution of a silver saltof an organic acid, such as the neo acids (particularly those having atleast seven carbon atoms) disclosed in U.S. Pat. No. 4,864,042 which isincorporated herein in its entirety. A hydrocarbon solvent is employed,such as toluene, cyclohexane, xylene, ethyl benzene, cumene or nonenewhich would normally be water free. Since water is considered to bedetrimental to the preparation of silver catalysts when the method ofthe invention is used, it should be present in no more than about 0.1vol % in the silver impregnating solution, preferably less than about0.01 vol %.

After impregnation with the silver salt the support is then activated inthe low temperature process as described, preferably on a moving belt inthe atmosphere as specified. The silver impregnated support is activatedin an inert atmosphere such as nitrogen, carbon dioxide or helium, at atemperature below 300° C. as recited and for limited periods.

After the low temperature activation the support may be impregnated withthe alkali metal if desired. It is the purpose of alkali metal to modifythe catalyst and raise selectivity while leaving the improved stabilityintact. When used the amount of the alkali metal on the finishedcatalyst is generally similar to those employed heretofore. Thus theamount deposited will be generally up to about 8×10⁻³ gew/kg catalyst,preferably up to about 7×10⁻³ gew/kg, and particularly about 1 to 6×10⁻³gew/kg (gew=gram equivalent weight). The alkali metals of the periodictable include sodium, lithium, potassium, rubidium and cesium. Forpurposes of the present invention, the latter three alkali metals areparticularly preferred, especially cesium, although sodium and lithiumare not necessarily excluded. The alkali metal salts are dissolved inalcohol solutions, preferably substantially free of water.

The improvement from the use of Cesium salt in a pure alcohol solvent,substantially free of water is believed to relate to the relatively poorsolubility of cesium salt in alcohol. In the absence of water in thealcohol solvent, the cesium compound, although poorly soluble, remainsevenly distributed through the solvent during evaporization and drying,hence is more evenly distributed over the silver catalyst. Preferablythe alkali metal impregnated catalysts are dried rapidly, e.g. one totwo minutes at high temperature, e.g. at least 100° C. up to 800° C.,preferably around 200° C. to 600° C. This may be readily achieved byusing a moving belt as described herein. The drying may be conducted inair or an inert gas.

Catalysts prepared by the procedures above have improved performance,especially stability, for use in the production of ethylene oxide by thevapor phase oxidation of ethylene with molecular oxygen. These usuallyinvolve reaction temperatures of about 150° C. to 400° C., usually about200° C. to 300° C., and reaction pressures in the range of from 0.5 to35 bar. Reactant feed mixtures contain 0.5 to 20% ethylene and 3 to 15%oxygen, with the balance comprising comparatively inert materialsincluding such substances as nitrogen, carbon dioxide, methane, ethane,argon and the like. Only a portion of the ethylene usually is reactedper pass over the catalyst and after separation of the desired ethyleneoxide product and the removal of appropriate purge streams and carbondioxide to prevent uncontrolled build up of inerts and/or by-products,unreacted materials are returned to the oxidation reactor.

The catalyst prepared by the process of the present invention has adifferent silver particle size and dispersion than catalysts prepared byconventional processes. In the standard activation step in which thesilver is reduced to its elemental form and deposited and dispersed uponthe surface of the support the support impregnated with the silversolution is subjected to an elevated temperature in the presence of agas containing oxygen, normally air. This results in very fine silverparticles and is shown in FIG. 1 which is a scanning electron micrograph(SEM) of a catalyst which was first activated in air at 500° C. Thesurface density of the particles is in the range of 110-125 particlesper square micron (ppμ²). Even when the temperature is reduced to 300°C. and air is used the particle density is about the same as shown inFIG. 2. Also when the higher temperature is used with an inertatmosphere the results are about the same as shown in FIG. 3 where thecatalyst was first activated at 500° C. in a nitrogen atmosphere.However, when the lower temperature, i.e., 300° C., is combined with aninert atmosphere such as nitrogen, the silver particles are much largerand less dense, e.g. in the range of about 10-70 ppμ² as shown in FIG.4.

The catalysts with the finer silver particle dispersion lose theirselectivity during use (over about 1000 hours). The same catalystshaving the 110-125 ppμ² initial dispersion end up with a silverdispersion of about 1-2 ppμ² as shown in FIG. 5 which is a SEM of acatalyst having an initial dispersion of 110-125 ppμ² after about 1000hours in an ethylene oxide reactor. A catalyst having the lower initialdispersion of about 10-25 ppμ², however, after about 1000 hours ends upwith a silver particle dispersion of 25 ppμ² as shown in FIG. 6.

The finished catalysts are then tested for activity and selectivity bycrushing and placing 36 grams in a micro reactor consisting of a 1/4inch stainless steel tube which is heated in a salt bath. A feed mixtureof 7% oxygen, 8% CO₂, 15% C₂ H₄, 70% N₂ is passed over the catalyst witha gas space velocity of 5500 hr⁻¹. The pressure is maintained at 300psig (21.69 bar) and the temperature between 200° C. and 300° C. asrequired to maintain an outlet concentration of 1.5 vol % (160 Kg perhour per m³ of catalyst) ethylene oxide. The activity of the catalyst isexpressed as the temperature necessary to maintain the outletconcentration at 1.50 vol % ethylene oxide, the lower the temperature,the more active the catalyst. The selectivity of the catalyst isexpressed as the mole % of the total ethylene converted to ethyleneoxide at the outlet concentration of 1.50 vol % ethylene. The stabilityof the catalyst is measured by the increase in temperature required tomaintain the ethylene oxide productivity.

EXAMPLE 1

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 5/16 inch cylinders.The support has a surface area of 0.55 m² /g, pore volume of 0.3 cc/g,and medium pore diameter of 1.5μ. A 95 parts of a cumene solution ofsilver neodecanoate, containing 26 wt % silver, was added to 225 partsof the hot support and the mixture was mixed for 20 minutes. Thedeposition of the silver was induced by heating the impregnated supportto a temperature that did not exceed 300° on a moving belt in a streamof nitrogen. The residence time of the catalyst in the heated zone wastwo minutes.

The catalyst was then impregnated for two hours at room temperature inan anhydrous ethanolic solution that contained 525 ppm cesiumbicarbonate. The catalyst was superficially dried by a stream ofnitrogen followed by heating on a moving belt at 200° C.

A sample of the catalyst was tested in a tube that is heated by a saltbath. A gas mixture containing 15% ethylene, 7% oxygen, and 78% inert(mainly nitrogen and carbon dioxide), was allowed to flow over thecatalyst under 300 p.s.i. The temperature of the reaction was adjustedin order to obtain ethylene oxide productivity of 160 kg/hr/m³ ofcatalyst. The results of the catalyst test are summarized in thefollowing table. Additionally comparative SEM's of the fresh catalystindicate the initial silver dispersion was 10-70 ppμ² and the silverdispersion of the catalyst after removal was 25-75 ppμ².

                  TABLE I                                                         ______________________________________                                        Life, hr     Temp., °C.                                                                       Selectivity, %                                         ______________________________________                                        150          230       82.3                                                   400          230       82.2                                                   700          231       82.3                                                   977          231       82.2                                                   ______________________________________                                    

A similar test was run for a catalyst of the type shown in FIG. 1 showedlower selectivity, 81.3% and a faster catalyst deactivation.

EXAMPLE 2

A catalyst was prepared in substantially the same manner and using thesame support as Example 1 except that the initial calcination wascarried out in a gas stream of nitrogen containing 2.5% oxygen. After100 hours in the reactor the selectivity was 82.0% and the reactortemperature was 228° C.

EXAMPLE 3 (Comparative)

The support used for this preparation was obtained from Norton Companyand was made primarily of α-alumina in the form of 5/16 inch cylinders.The support has a surface area of 0.55 m² /g pore volume of 3 cc/g, andmedium pore diameter of 1.5μ. A 95 parts of a cumene solution of silverneodecanoate, containing 26 wt % silver, was added to 225 parts of thehot support and the mixture was mixed for 20 minutes. The catalyst wasprepared using activation with air at 500° C. and was impregnated withcesium hydroxide solution in water/alcohol solvent, which wassubsequently dried with vacuum. The catalyst was tested under the samecondition as in example 1. After 150 hours of reaction time theselectivity to ethylene oxide was 80.9% and the reaction temperature was232° C. The catalyst's performance did not improve with longer reactiontime.

The invention claimed is:
 1. A catalyst for the vapor-phase oxidation ofethylene to ethylene oxide comprising:a porous support having a surfacearea of about 0.1 to 2.0 m² /g and a porosity of about 0.1 to 1.0 cc/g;and 3-20 wt % silver dispersed on said support such that there are from10 to 70 particles of silver per square micron on the fresh catalyst. 2.The catalyst according to claim 1 wherein the silver is dispersed onsaid support such that there are from 10 to 20 particles of silver persquare micron.
 3. The catalyst according to claim 1 further comprisingup to about 8×10⁻³ gew/kg of alkali metal on said support.
 4. Thecatalyst according to claim 3 wherein said alkali metal is cesium. 5.The catalyst according to claim 1 wherein said support comprisesalpha-alumina.
 6. The catalyst according to claim 5 wherein said supportfurther comprises up to about 15 wt % silica.
 7. The catalyst accordingto claim 1 wherein said support has a surface area of 0.4 to 1.6 m² /gand a porosity of about 0.2 to 0.7 cc/g.
 8. The catalyst according toclaim 6 wherein said support has a surface area of 0.4 to 1.6 m² /g anda porosity of about 0.2 to 0.7 cc/g.
 9. The catalyst according to claim7 further comprising up to about 8×10⁻³ gew/kg of an alkali metal onsaid support.
 10. The catalyst according to claim 9 wherein said alkalimetal is cesium.
 11. The catalyst according to claim 10 wherein thecesium content is about 1 to 6×10⁻³ gew/kg.
 12. A catalyst for thevapor-phase oxidation of ethylene to ethylene oxide comprising:a porousalpha-alumina/silica support having a surface area of about 0.4 to 1.6m² /g and a porosity of about 0.2 to 0.7 cc/g; 8-13 wt % silverdispersed on said support such that there are initially from about 10 to20 particles of silver per square micron; and 1to 6×10⁻³ gew cesium/g onsaid support.